What happened
On July 22, 1977, a Piper PA-32-300, registration HB-OMB, was conducting a sightseeing flight near Zurich with five passengers on board. During the climb phase, at an altitude of approximately 1,000 meters near Birmensdorf, the pilot experienced engine vibrations followed by a complete engine failure. The pilot immediately attempted to restart the engine by checking the fuel selector and activating the booster pump, but the engine would not restart.
Due to the hilly, heavily vegetated, and developed terrain, finding a suitable landing site was difficult. The pilot selected a field northwest of Hedingen. During the final approach, the pilot encountered high-voltage power lines and a stream crossing the intended path. While the pilot successfully cleared these obstacles, the aircraft touched down in a soft field approximately 1.5 meters past a farm track. The landing gear sank into the soft ground and collapsed, causing the aircraft to skid and rotate before coming to a stop.
The investigation
The investigation focused on the mechanical failure of the Lycoming IO-540-K1A5 engine. Upon inspection, investigators found that the camshaft was not rotating with the crankshaft. Disassembly of the engine revealed several critical failures:
- A gear on the crankshaft had become loose, and its drive pin had sheared.
- The camshaft drive gear had broken teeth, with fragments found in the oil pan.
- The drive mechanism for the left magneto was defective, and the housing for the roller bearing was deformed.
- The snap coupling of the left Bendix S 1200 magneto was destroyed.
Metallurgical analysis by EMPA revealed that the magneto coupling pins and pawls showed extreme wear, creating significant internal clearance. This wear led to increased mechanical stress and the formation of hardness cracks in the hardened edge zone of the pawls.
Findings
- The primary cause of the engine failure was a break in the left magneto's snap coupling.
- A fragment from the broken coupling entered the camshaft drive, causing the camshaft to stop rotating and resulting in an immediate loss of engine power.
- The breakage was driven by excessive wear within the magneto coupling system, which created abnormal mechanical loads.
- The pilot's decision-making during the emergency was appropriate, and the aircraft's weight and balance were within limits.
Safety action
Following the accident, the manufacturer, Bendix Electrical Components Division, issued a service bulletin in March 1978. This bulletin required the disassembly and inspection of this magneto type for abnormal wear at 1,000 operating hours. This was subsequently adopted as an Airworthiness Directive by US authorities and implemented by the Swiss Federal Office of Civil Aviation.